Transcriber's note:
Spelling and punctuation inconsistencies have been harmonized.
Obvious printer errors have been repaired. Italic text has been
marked with _underscores_. The symbol for male has been
rendered as [M] and for female as [F].
The Cambridge Manuals of Science and
Literature
EARTHWORMS AND THEIR ALLIES
CAMBRIDGE UNIVERSITY PRESS
London: FETTER LANE, E.C.
C. F. CLAY, MANAGER
[Illustration]
Edinburgh: 100, PRINCES STREET
London: WILLIAM WESLEY & SON, 28, ESSEX STREET, STRAND
Berlin: A. ASHER AND CO.
Leipzig: F. A. BROCKHAUS
New York: G. P. PUTNAM'S SONS
Bombay and Calcutta: MACMILLAN AND CO., LTD.
_All rights reserved_
EARTHWORMS AND
THEIR ALLIES
BY
FRANK E. BEDDARD
M.A. (Oxon.), F.R.S., F.R.S.E.
Cambridge:
at the University Press
New York:
G. P. Putnam's Sons
1912
Cambridge:
PRINTED BY JOHN CLAY, M.A.
AT THE UNIVERSITY PRESS
_With the exception of the coat of arms at the foot, the design on
the title page is a reproduction of one used by the earliest known
Cambridge printer, John Siberch, 1521_.
PREFACE
The importance of earthworms in questions relating to geographical
distribution is so universally admitted that it seemed to me
convenient to embody in a short volume the principal facts.
It became necessary in order to accomplish this task in an adequate
fashion to preface the distributional facts with some anatomical and
zoological data. I have reduced this section of the book to a minimum
and I trust that the illustrations will enable the reader, who is
not specially acquainted with the structure of these animals, to
obtain an idea of their general features and variability in external
character and internal anatomy. While the use of technical terms is
inevitable in presenting such details, it will be found, I think,
that a reference to the figures will render them intelligible.
Since this volume mainly deals with the phenomena of distribution, I
have included in my survey nearly all of the usually admitted genera
of worms, particularly of the terrestrial forms, which are in the
light of our present knowledge the more important in considering this
subject.
F. E. B.
ZOOLOGICAL SOCIETY OF LONDON.
_December_, 1911.
CONTENTS
CHAP. PAGE
I. Structural and Systematic 1
II. Mode of Life 43
III. The external features of Earthworms and their
relation to habit and environment 59
IV. Sense Organs and Senses of Earthworms 64
V. Relative frequency of Earthworms in different
regions of the World 70
VI. Peregrine forms 96
VII . The Earthworms of Oceanic Islands 109
VIII. Movement and Migration among Earthworms 113
IX. The Geographical Distribution of Earthworms 129
List of Literature referring to Earthworms 144
Index 146
CHAPTER I
STRUCTURAL AND SYSTEMATIC
The group of segmented, bristle-bearing, worms, termed Oligochaeta by
zoologists, comprises what are popularly known as earthworms together
with certain forms, inhabitants of ponds, lakes and rivers, which are
not so well known as to have received a more distinctive name than
merely 'worms.' Their next allies are apparently the leeches and--a
little more remote--marine bristle-bearing worms termed Polychaeta;
the three groups, together with perhaps a certain number of other
forms belonging to smaller groups, constitute the Annelida which are
a distinct and separate assemblage of invertebrate animals.
The most interesting features about these Oligochaetous worms
are their very great anatomical variation and the facts of their
distribution over the globe. Their importance as geological agents
in levelling the ground was made known a long time ago by Darwin,
and that aspect of earthworms has remained in much the same position
as Darwin left it. We shall concern ourselves here only with the
structure, habits, and range of the earthworms and their immediate
allies, the aquatic Oligochaeta. These three aspects of the animals
dovetail into each other more thoroughly than is the case with some
other groups. This is due to the fact that they have of late years
been very thoroughly studied from the anatomical and distributional
side. So lately as 1889, M. Vaillant in a very comprehensive treatise
was only able to enumerate 369 species, of which a large number were
but incompletely differentiated, and some are no longer admitted.
There are at the moment of writing perhaps 1500 species, the vast
majority of which are well known owing to careful investigation.
Furthermore there are but few parts of the world, and these are
not of large area, from which earthworms at any rate have not been
gathered. Though there can be no doubt that a very considerable
number of species await discovery, it would seem that we are in
possession of information which is not likely to be seriously
affected by future researches.
THE ANATOMY OF EARTHWORMS.
Although it is not contemplated to make the present volume a guide
to the structure of this group of worms, it is necessary to give
some little anatomical sketch of the group in order first of all to
illustrate their diversity of structure, secondly to give reasons
for the classification of them, and thirdly to enable the reader to
realise certain structural details which it is absolutely necessary
to give some account of in order to explain other matters.
It is for example impossible to attempt any account of the fitness of
some of these animals for their terrestrial life and of others for an
aquatic life without treating of anatomy to some extent.
I shall take one particular species as a type and indicate later the
principal divergencies shown by other forms. According to the general
opinion among those who have studied the Oligochaeta I take as a
representative form a Megascolecid (this and the other families are
dealt with _seriatim_ on p. 14 et seq.), as this group is presumed to
be the oldest, and within that group a representative of the genus
_Notiodrilus_ which is with some reason held to be the most primitive
genus in the group. Finally I have no particular reason for selecting
the species _Notiodrilus tamajusi_ except that there happens to be a
longer and fuller description of it than of many.
[Illustration: Fig. 1. _Notiodrilus tamajusi._ The worm shown
from the ventral surface. About natural size. (After Eisen.)]
This earthworm is a native of Guatemala and is some six inches in
length with a diameter of perhaps a quarter of an inch. The front
part of the body is thicker than posteriorly. The body will be seen
to be divided into some 218 rings by circular furrows which run right
round the body. These divisions are termed segments or somites. At
the head the mouth is surrounded by the first of these, and on the
dorsal surface of that segment is a projection like an incomplete
segment which is known as the prostomium. From the XIIIth segment to
the middle of the XXth the body has a different appearance, and this
region is known as the clitellum. Each of the 218 segments of the
body except the first, and possibly one or two at the hinder end, is
furnished with eight minute projecting bristles, the setae; these are
disposed in pairs and all lie upon the ventral aspect of the worm.
The movement of these by special muscles aids in locomotion.
[Illustration: Fig. 2. The same species; first two segments and
prostomium shown from ventral surface, I prostomium, II first
segment without any setae, III second segment with its four pairs
of setae (the dorsalmost seta of each outer pair is not visible
in this view).]
[Illustration: Fig. 3. A section through the body of the same
species showing the ventral position of the pairs of setae.
(After Eisen.)]
An examination with even a hand lens shows a number of external pores
which are important. Anteriorly there is the mouth which is overhung
by the prostomium referred to above. At the extreme hind end--and
surrounded by the last segment of the body--is the vent. Along the
middle line of the back are a series of pores, one just at the very
anterior edge of each segment, through which, when the worm is dried
and then slightly pressed, liquid is seen to be ejected. These are
called the dorsal pores and they belong one to each segment with the
exception of the first seven, or--in some cases--more, segments.
In front of one or other of the pair of setae which is situated
most laterally, _i.e._ furthest from the ventral median line, is an
orifice on each side in all but the first one or two segments of the
body. These paired pores are the external outlets of the excretory
organs frequently termed on account of their regular repetition
with the segments 'segmental organs,' but more conveniently to be
named nephridia. In the clitellar region and in fact on each of the
segments XVII, XVIII, XIX are a pair of pores of which those on the
XVIIIth segment are the least conspicuous. The large pair of pores on
each of segments XVII and XIX occupy the position of the ventralmost
pair of setae, which are here absent, or rather replaced by a very
long curved and ornamented seta, which projects out of the orifice.
These two pairs of pores are the outlets of the prostatic glands as
they have been termed. The minute pair of pores on segment XVIII do
not take up the position of the ventral setae; for these are present
and to the inside of each pore. A groove, shaped something like a
reversed 3 or the Greek letter [Greek: S], connects the orifices of
each side of the body, the middle part of the groove, where the two
semicircular halves of which it is composed meet, coinciding with the
minute pores on segment XVIII which are the orifices of the sperm
ducts.
[Illustration: Fig. 4. The same species. Ventral view of segments
XVI-XX (numbered in the figure) which form the clitellum, the
posterior boundary of which is shown by a curved line on segment
XX. The figure will be understood from the annexed description.]
On segment XIV are a pair of very minute pores a little in front of
the ventralmost setae and thus very near together. These are the
openings of the oviducts. Finally, near to the anterior border-line
of segments VIII and IX and on a line with the ventral pair of setae
is a pore on each side through which the cavity of the spermathecae
reaches the exterior.
[Illustration: Fig. 5. A longitudinal section through the middle
of the first nineteen segments of the body of _Notiodrilus
vasliti_ (a species very closely allied to that described in the
text), the segments are numbered I, II, &c. _D.v._ dorsal blood
vessel, _G_ gizzard, _H_ hearts, _oe_ oesophagus lying in front
of gizzard behind which another tract of still narrow oesophagus
is seen opening into _Int._ intestine. The whole alimentary canal
is supported by the intersegmental septa (_Pr.c._) between which
is the system of spaces forming the coelom, _ov.d._ pore of
oviduct, _Splh._ orifices of spermathecae, [M] orifice of sperm
duct, _o_ ovary, _t_ spermaries. (After Eisen.)]
So much then for the external characters of our worm. We next turn
to the internal anatomy. When the worm is opened by a longitudinal
section from end to end, and the two flaps of skin are turned
outwards and pinned down, the internal structure is almost completely
revealed. Running from end to end is seen the alimentary canal;
the general cavity of the body (coelom) in which it lies, as do of
course the other organs to be enumerated, is seen to be divided by
cross divisions, the intersegmental septa, into a series of chambers
which correspond with the external division into segments. The
septa are in fact inserted on to the body-wall along the furrows
which mark the divisions between adjacent segments. Anteriorly
the large pharynx is responsible for confusing the arrangement of
the septa, which become subdivided and fused or are prolonged a
greater way backwards and thus present a less obviously segmental
disposition. Certain of the more anteriorly placed of these septa
are much thicker than the rest. This is the case with the septa
which separate segments V to XII. The alimentary canal is perfectly
straight and runs in the middle line, being supported by the septa
which it perforates. The mouth leads into a buccal cavity which later
becomes the pharynx, a portion of the tube which is much thickened
by muscular walls dorsally. Then follows a very short section of
the oesophagus and in the fifth segment this becomes the gizzard,
a very characteristic organ with thick muscular walls quite smooth
and with a very thick lining of structureless membrane. After this
is a narrower tube, the rest of the oesophagus. Into this open in
each of segments VII, VIII, IX a pair of calciferous glands; these
are diverticula of the gut with much folded walls, the cells of
which secrete carbonate of lime. In the XIIth segment or so, the
oesophagus suddenly widens out to form the intestine which runs as
such to the end of the body. This wider tube has a ridge running
along its dorsal side, the typhlosole. Along the dorsal surface of
the intestine and the oesophagus is seen a red tube, contractile
during the life of the worm, which is the dorsal blood vessel and
whose contained blood is coloured red, as is the blood of vertebrated
animals, by haemoglobin. But in the earthworm the colouring matter
is not situated in corpuscles as in the vertebrate. The dorsal
vessel is connected by a few pairs of equally contractile transverse
trunks with a ventral vessel which is not contractile. There are
other branches of these main longitudinal trunks and some minor
longitudinal trunks which we shall not stop to describe further.
The nervous system of the worm consists of a pair of ganglia which
lie above the gut in the third segment; they are connected by a
commissure running round the gut with a chain of closely fused pairs
of ganglia, one for each segment to the very end of the body. In each
of the segments, except the first two or three, there are a pair of
excretory organs known as nephridia; these are essentially coiled
glandular tubes opening on to the exterior by the regularly placed
pores already referred to in considering the external characters. The
tube ends in a funnel-shaped, and therefore dilated, mouth, which
opens into the segment in front of that which contains the rest of
the organ; a nephridium therefore lies in two segments. The only
other important organs which are left for consideration are those
devoted to the reproduction of the species. The essential organs are
the spermaries and the ovaries. Of the former there are two pairs
of minute whitish bodies which lie in segments X and XI on either
side of the nerve cord attached to the anterior septal wall of their
segments. The ovaries are not in the following, but in the XIIIth,
segment, and occupy an identical position in that segment. A short
tube with a funnel or trumpet-shaped and wide orifice opens into the
cavity of the XIIIth segment opposite to each, and, perforating
the septum, opens on to the exterior on the XIVth segment. A similar
but larger and more folded pair of trumpet-shaped funnels opens in
the same way opposite to each spermary. But in this case the two
tubes of the sperm ducts run backwards for some way and those of
each side after joining open on to the XVIIIth segment by the pores
already mentioned. On the XVIIth and XIXth segments open two glands
which are long and tubular in form and much coiled. These are the
spermiducal glands and each opens in common with a muscular sac
containing the long and ornamented seta referred to in describing
the various external orifices. It will be noticed that the sperm
duct has no direct connection with these glands but only indirectly
through the external gutter which connects the three male orifices
of each side of the body. Segments IX-XII inclusive contain certain
sacs which depend from, and are formed as outgrowths of, the septa
of those segments. These are the sperm sacs in which the male germ
cells undergo their development. A corresponding body (but very much
smaller) is sometimes found in relation to the ovary but has not
been actually described in the particular species dealt with here.
Finally, in segments VIII and IX are a pair (that is four altogether)
of roundish sacs, with two or three minute diverticula, known as the
spermathecae. In the diverticula of these sacs are stored the sperm
derived from another individual.
[Illustration: Fig. 6. Genital region of _Maoridrilus dissimilis_
(in which species the different organs are essentially as in
_Notiodrilus tamajusi_). The oesophagus is cut away between the
Xth and XVIIIth segments to display the various organs. _n_
external orifices of nephridia which alternate in different
segments; the one lettered opens in front of dorsalmost pair of
setae, that of segment XVI opens in front of ventralmost pair,
_p_ coiled glands opening on to XVIth and XIXth segments, _sc._
sacs containing long seta, associated with these glands, _t_
spermary of segment; another pair in an exactly similar position
in segment XI. Behind spermaries are funnel-shaped openings
of sperm ducts which are seen running along the body to their
external orifice on XVIIIth segment. _v.s._ sperm sacs behind the
posterior of these and in segment XIII is seen the large ovary
corresponding in position to the spermary and opposite to each
ovary the oviduct. (x 3.)]
[Illustration: Fig. 7. A spermatheca of _Notiodrilus tamajusi_.
The external orifice is shown at _o_; above this are the
diverticula, _s_ is a portion of an intersegmental septum. (After
Eisen.)]
This completes the general sketch of the structure of _Notiodrilus
tamajusi_ which we have selected as a type. In this same genus are a
large number of species which differ from that selected in various
small structural points. Thus in _N. annectens_ (Beddard), a species
from New Zealand, the spermaries and ovaries are attached to the
posterior, instead of to the anterior, wall of their segments, and
there are neither calciferous glands nor modified setae upon segments
XVII and XIX. In all essentials however the two types agree and are
thus to be looked upon as referable to the same genus. Starting from
the structure of these types we may now sketch in quite a brief way
the main divergencies of structure shown in the group of Oligochaeta.
We shall naturally begin with the family Megascolecidae of which a
type has just been described.
Within the limits of the same sub-family as that which contains
_Notiodrilus_, _i.e._ the Acanthodrilinae, the changes of structure
affect all the principal organs of the body except the nervous
system, but are not very large and vary from genus to genus. They
are mainly perhaps in the direction of reduction and simplification.
Thus in _Chilota_, _Maheina_ and _Yagansia_ the spermaries are
reduced to one pair in either the Xth or XIth segment, while in
_Yagansia_ one pair of spermathecae and of spermiducal glands have
also disappeared. In _Microscolex_ the spermaries remain normal, but
one pair of spermathecae and of spermiducal glands have disappeared,
the remaining organs of these series being in the IXth and XVIIth
segments respectively. In _Microscolex_, _Chilota_ and _Yagansia_,
moreover, there is a further degeneration in the disappearance of the
calciferous glands. These glands are often absent and sometimes less
developed in the New Zealand _Maoridrilus_, which is otherwise not a
degenerate form and differs characteristically from _Notiodrilus_
by the fact that the paired nephridia alternate in position in
successive segments, being now in front of the dorsal, and in other
segments in front of the ventral, pairs of setae. While these genera
are somewhat degenerate, the New Zealand _Plagiochaeta_ has undergone
specialisation in an upward direction. For the setae of each segment
are increased to a large number much exceeding eight.
It is not a long step to the sub-families Diplocardiinae and
Trigastrinae. In the first of these, an American race confined to the
northern and central parts of that continent, the male pore shows
a tendency to move backwards, being situated on any of segments
XVIII-XXI. The two spermiducal glands follow it, but are always
placed one pair in front and one behind, as in _Notiodrilus_. In this
group we get a new feature of specialisation in the duplication or
triplication of the gizzard.
So too with the Trigastrinae where there are either two or three
gizzards; but in this sub-family another modification has become
apparent. The paired nephridia have disappeared and their place
is taken by several, often quite numerous, pairs of much smaller
nephridia called on that account 'micronephridia' instead of
'meganephridia.' To this sub-family belong the especially African but
also American and Malayan _Dichogaster_, whose name is derived from
the important fact that it possesses two gizzards.
Not far off is to be placed another sub-family, that of the
Octochaetinae, which is New Zealand and Indian in range, the
intermediate countries being, strange to say, not populated by this
race of Oligochaeta. The group contains several genera of which
_Octochaetus_, _Eutyphoeus_, and _Dinodrilus_ are the best known.
All these worms agree in the main features of their anatomy with
_Notiodrilus_; but they have diverged in different directions. Thus
in _Octochaetus_ the typical two pairs of gonads and glands belonging
to the generative system have been retained, while the nephridial
system consists of micronephridia; in _Eutyphoeus_, one pair of
spermiducal glands has disappeared, and as a general rule the species
of this genus have only one pair of spermaries and the corresponding
pair of sperm ducts. They are close to _Octochaetus_. The third
genus mentioned, _Dinodrilus_, is a New Zealand form specialised in
possessing 12 setae in each segment. Otherwise it is not far removed
from _Octochaetus_.
[Illustration: Fig. 8. Ventral view of _Eutyphoeus masoni_. _p_
papillae, [M] male pores, [F] oviduct pores. (x 3.)]
A fifth sub-family is also easily referable to the type whose
structure has been dealt with as a preliminary to the present survey.
That sub-family is the Ocnerodrilinae which is American and African
in range. These worms are somewhat degenerate in comparison with
their allies. Thus the calciferous glands are reduced to a single
pair or to a single gland in the IXth segment, the nephridia though
regular and paired have no covering plexus of blood vessels, and
the worms themselves are slender and delicate, being indeed often
aquatic in habit. The spermiducal glands, which are as in the former
sub-families independent of the sperm ducts though sometimes opening
in common with them into a short pocket-like ingrowth of the skin,
are reduced in their minute structure and much simpler than in the
other types.
The genus _Kerria_ is the least reduced perhaps. It has the male
pores on segment XVIII with a pair of spermiducal glands on the
segments preceding and following this in the typical Acanthodriline
fashion. There are two pairs of spermathecae in VIII and IX,
but the spermaries are reduced to one pair in X. The gizzard is
present. OCNERODRILUS is a little further reduced from this last.
The gizzard has gone; there is but one pair of spermiducal glands
(as a rule) opening in common with the extremity of the male duct
on to segment XVII; the spermathecae also are reduced to one pair,
but there are two pairs of spermaries. The African _Nannodrilus_ is
more robustly built. There are two gizzards, the male duct opens
into a muscular pouch, into which also open one of the two or three
pairs of spermiducal glands. There are two or three other genera
and sub-genera not showing any great divergencies from the range of
structure indicated in briefly defining those enumerated above.
Finally, we have the largest of all the sub-families of this family,
viz. that of the Megascolecinae. These worms are mainly tropical in
range and also mainly found in the Indo-Australian portion of the
world. In them the sperm ducts open in common with the usually single
pair of spermiducal glands and prevalently upon the XVIIIth segment.
The glands moreover have not always, though they often have, the
tubular form shown in all the sub-families hitherto considered. In
many forms they are branched and lobate glands, and if there are two
pairs one may be of one type and the other of the second and derived
type, as for instance in _Megascolex ceylonicus_. Furthermore, it is
much commoner among the genera of this sub-family for the setae to
become numerous and to spread right round the segment; this condition
is seen in the genera _Pheretima_, _Megascolex_, _Diporochaeta_,
_Perionyx_, _Plionogaster_. The spermathecae also are commonly more
than the typical two pairs of the forms already considered, and in
certain species (for example _Pheretima hexatheca_) there are as many
as six pairs of those organs which are moreover--and in this they
resemble the majority of species of the last sub-families--nearly
always furnished with a diverticulum or diverticula. The nephridia
are either paired or numerous and these various characters allow of
the sub-family being split up into sixteen genera or thereabouts.
As an example of another type of organisation and as contrasting with
_Notiodrilus_ we may now briefly describe the structure of the genus
_Pontoscolex_ (better known as _Urochaeta_), a member of the American
and African family Geoscolecidae.
The worm is some four inches long and composed of as many as 212
segments. Each of these except the first has eight setae which for
the first few segments of their occurrence are disposed in four pairs
in the usual fashion. Behind this point however the setae cease to
present this symmetrical arrangement and are irregularly disposed
so that a given seta is not in the same line with the corresponding
seta of the segments in front or behind. There is thus no region of
the body which has not a seta implanted upon it; and the effect is
therefore comparable to the condition obtaining in those worms, such
as _Pheretima_, where circles of numerous setae are met with. There
are however only eight in a given segment. The clitellum extends
from segment XV to XXII or XXIII and is developed only dorsally and
laterally. It has setae like the rest of the body; but those upon
the clitellum are longer and more markedly ornamented than are those
of the body generally. The latter are not only sculptured with
fine ridges but are bifid at their free extremity. The prostomium
is often apparently completely absent. It is however really present
but is retractile. As to the pores which are visible externally the
dorsal pores are completely absent. The pores of the nephridia lie in
front of the dorsal pair of setae or in a line corresponding to the
position of those setae where the arrangement has become irregular.
The spermathecal pores are three pairs and are in the very front
of segments VII, VIII, IX. The male pores, very inconspicuous, lie
on the ventral side of segment XXI just within the clitellum. The
oviducal pores are on segment XIV.
As to internal anatomy the general plan of the segmentation shows
no great differences. Certain septa only show a difference, _i.e._
those separating segments VI-XI which are specially thickened. In the
alimentary canal a gizzard in segment VI is to be noted and three
pairs of calciferous glands in segments VII-IX. The nephridia are
paired structures and commence early. The first two or three segments
are occupied by a pair of large glands opening into the buccal
cavity which are apparently a slightly modified pair of nephridia
and are generally termed 'peptonephridia' since they are associated,
as it would appear, with the function of alimentation and are not
purely excretory organs. There is but a single pair of spermaries in
segment XI, and of ovaries in segment XIII. The sperm ducts open on
to the exterior in the position already mentioned and they are not
associated at their pore with any glands comparable to spermiducal
glands. A pair of sperm sacs depend from segment XI and traverse a
considerable number of segments, being thus long and tongue-shaped
instead of short and limited to one segment. The spermathecae are
three pairs of elongated sacs in segments VII-IX, without any
diverticula at all.
It will appear therefore that many and considerable differences
divide _Pontoscolex_ from _Notiodrilus_ and indeed from all of the
Megascolecidae whose structure has been touched upon in the foregoing
pages. The most important of these are the ornamented setae and their
arrangement and the modification of the setae upon the clitellum:
the absence of diverticula to the spermathecae: the absence of
terminal glands associated with the male ducts. Although taken in
their entirety these characters are distinctive of the American
Geoscolecidae (sub-family Geoscolecinae), there is no one of them
which is not to be found in some Megascolecid. Thus the subgenus
_Ilyogenia_ (of _Ocnerodrilus_) has sometimes no spermiducal glands:
the genus _Perionyx_ has spermathecae without diverticula in some
species, and other genera of Megascolecinae are in a like condition.
The setae of _Dichogaster_ are sometimes ornamented, while in
_Pheretima houlleti_ the clitellar setae are different from those
upon the other segments.
We can in fact only define the family Geoscolecidae by an assemblage
of characters which are mainly these: dorsal pores absent, only a
few in the neck region being occasionally present; setae generally
ornamented, those of the clitellum being larger and more marked than
the others; spermathecae without diverticula; often instead of a
pair of those pouches in the segment a large number of very small
sacs, as in _Microchaeta_, _Kynotus_. Sperm ducts without terminal
glandular or muscular sac, except in a few cases; setae always eight
in a segment except in the genus _Periscolex_ which has acquired
the 'perichaetous' condition. The range of variation shown in the
anatomy of the Geoscolecidae will be best taken in connection with
the several sub-families into which it has been subdivided. In the
first of these, the Geoscolecinae, no great differences divide the
genera from that selected as the type, viz. _Pontoscolex_. The long
sperm sacs attain to an extraordinary length in _Trichochaeta_ (or
_Hesperoscolex_) where the single pair extends through no less than
109 segments. Though as a general rule the sperm ducts open directly
on to the exterior they do so through the intermediary of a large
pouch in _Glossoscolex_ (= _Titanus_). In _Onychochaeta_ the setae on
the last segments of the body are very much enlarged and thus form a
more effective means of holding on to the soil than is possessed by
other species.
The sub-family Hormogastrinae which contains but a single genus
_Hormogaster_ is remarkable for being limited in range to the
Mediterranean coasts. The genus is mainly distinguished by
possessing three gizzards; otherwise it is not very different
from the sub-family just described. The African and Madagascar
forms are associated (together with a few Asiatic forms) into a
third sub-family Microchaetinae. These worms frequently possess a
considerable number of very small spermathecae in segments XII, XIII
or thereabouts instead of the usual paired arrangement. They have too
very often glands connected with the enlarged setae already mentioned
which are however (in the genus _Kynotus_ at any rate) usually in
front of the clitellum. The latter organ, contrary to what we find
among the Geoscolecinae, is often behind the point of orifice of the
male pores. This is so with _Kynotus_.
[Illustration: Fig. 9. Ventral view of _Polytoreutus
Kilindinensis_, head end (x 2). _p_ papillae, [M] male pore, [F]
female pore.]
The last sub-family, Criodrilinae, has but three genera _Criodrilus_,
_Sparganophilus_ and _Alma_. These worms do not show any very marked
differences from other Geoscolecids. _Alma_ is noteworthy for the
facts that the male pores are borne upon long processes of the body
which bear specially modified setae and that one species at any rate
has gills.
[Illustration: Fig. 10. Ventral view of _Polytoreutus finni_,
head end (x 2), lettering as in fig. 9.]
Another type of structure is offered by the Eudrilid earthworms
which form rather a restricted family. These worms are as a rule
quite easy to distinguish by their external characters only. For the
apertures of the spermathecae and sperm ducts are apt to be very
large and conspicuous. They are also generally unpaired, a character
which is however not confined to the Eudrilidae; for there are
Megascolecids, such as _Fletcherodrilus_, and Geoscolecids in which
the same unpaired character occurs. The principal feature of the
family is that the ovaries are commonly enclosed in sacs--comparable
to the sperm sacs which frequently envelop the spermaries in
other earthworms--and that these sacs not only contain the mouths
of the oviducts but are directly continuous with the single or
double spermatheca. This is usually a large sac, always single or
consisting of one pair only, which opens on to the exterior close
to the oviducal pores; these spermathecae in the Eudrilidae are not
comparable to the spermathecae of other earthworms; for they are in
a way comparable to the sperm sacs, being formed as outgrowths of
the septa. There is some variation of structure within the family.
In a number which are associated into a sub-family Eudrilacea there
are two paired calciferous glands and a single unpaired one, while
the paired nephridia open by a large pore on to the exterior. In a
parallel sub-family, the Pareudrilacea, the calciferous glands are
apt to be more numerous and have a totally different structure: they
have been apparently converted into non-digestive glands bearing some
relation to the vascular system. The nephridia moreover do not open
on to the exterior by single pores, but form a network within the
thickness of the body wall and then open by numerous pores. There is
however no resemblance here to the micronephridia of _Dichogaster_
and other Megascolecids. In _Libyodrilus_ (as an example of the
Pareudrilacea) each nephridium forms a network out of the duct
leading to the exterior. In the interior of the body a series of
paired meganephridia are visible.
[Illustration: Fig. 11. Organs of reproduction of _Eudriloides
durbanensis_.]
The earthworms of Europe belonging to the family Lumbricidae
offer again a rather different type of structure, which is more
reminiscent of the Geoscolecidae than of the Megascolecidae or
Eudrilidae. In this family there are no glands appended to, or in
the neighbourhood of, the orifices of the sperm ducts, such as are
found in the other forms. As in the Geoscolecidae the clitellum is
furnished with setae somewhat different in form from those which
deck the body generally. These setae are never more than eight in a
segment. Dorsal pores (absent in Geoscolecidae and in Eudrilidae)
are invariably present. The spermathecae are without appendices and
nearly always simply paired, though rarely we get numerous much
smaller spermathecae in a single segment, as in _Kynotus_ among the
Microchaetine Geoscolecids. Internally the most striking feature of
this family is to be seen in the position of the gizzard at the end
of the oesophagus and at the beginning of intestine. The apertures
of the male pores are--save for two or three exceptions where they
are further forward--invariably upon the fifteenth segment, and
the clitellum, often very long, usually begins behind this point,
features which are also seen in _Kynotus_.
Finally we have the Moniligastridae which differ from all the
types hitherto considered in a few rather important particulars.
These worms are named on account of the fact that they possess
several gizzards upon the oesophagus, a character which is however
met with in the Megascolecid genus _Plionogaster_ and in certain
Eudrilids, _e.g._ _Hyperiodrilus_. The main peculiarity of the
family is that the sperm ducts are very short and open on to the
next segment to that which contains the spermaries, as in the
water-living Oligochaeta generally. The terminal sac into which the
male ducts open is moreover rather like that of such a family as the
Lumbriculidae.
THE AQUATIC FAMILIES OF OLIGOCHAETA.
It would seem to be quite possible that when the fresh waters of the
world have been as well examined for Oligochaeta as have so many
parts of the land areas, the number of purely aquatic Oligochaeta
will be found to equal those inhabiting the land. In any case we
are quite justified at the present moment in stating that there are
rather more families of these smaller Oligochaeta than there are of
the bulkier terrestrial forms. But while there are certainly seven or
eight distinct families, these do not between them contain at present
so many genera as do the fewer families of earthworms; and the number
of species of the latter that are known to science enormously exceeds
that of the 'Limicolae' as the fresh-water worms were at one time
called in common. The fact that there are purely marine forms of
these water worms was hardly appreciated at the time that the term
Limicolae was used; now however we are acquainted with a few such
forms, and even with some which live at will in either fresh, salt,
or brackish water. Of these something will be said later.
These forms have also been collectively treated of as Microdrili, a
term which expresses the undoubted fact that they are all of small
size and sometimes even minute; others however reach the dimensions
of the smaller species of earthworms. There are a certain number of
characters shared by the various families which may be considered
first of all, before dividing them into their several subdivisions.
These aquatic Oligochaetes are usually tender and transparent, the
muscular layers of the body wall being much reduced as compared with
the tougher terrestrial forms. The clitellum is also thinner and
consists of a single layer of cells only, thus contrasting with the
double layered clitellum of earthworms. As a rule the alimentary
tract is simplified, there being no gizzard or glandular appendices
of the oesophagus comparable to the calciferous glands of most
earthworms. But this rule is not without exceptions; for we find in
_Haplotaxis_ a gizzard occasionally developed, and in the remarkable
genus _Agriodrilus_ from the Baikal lake a continuous gizzard
formation along the oesophagus, while the Enchytraeidae may show
something very like calciferous glands: and even a Tubificid, called
by Pierantoni _Limnodriloides_, has a pair of diverticula of the gut.
Other internal organs show certain points of likeness in all or in
the great majority of the aquatic families. Thus the nephridia are
without a plexus of blood capillaries surrounding them, a state of
affairs which also occurs in some of the slender Ocnerodrilinae
among the earthworms. These paired organs also are very frequently
not found in the anterior segments of the body and these include
also as a general rule the segments in which the reproductive
elements are formed. Save for an exceptional case among the genera
of Enchytraeidae the dorsal pores are not found among the Limicolae,
but in some cases at least a single pore, the head pore, is found.
The sperm ducts, which among earthworms usually (and indeed always
save in the anomalous Moniligastridae) traverse a considerable
number of segments on their way from the internal opening into the
body-cavity to the external pore, do not show the same phenomenon
among the Limicolous Oligochaeta. They are sometimes indeed limited
to a single segment, that is to say the funnels and the external
pore lie in one segment. In other cases they open on to the exterior
in the segment next to that which bears the funnel, and it is only
rarely that they traverse more than one segment. There are also
points of difference of general applicability to be noted in the
sperm sacs and egg sacs. The latter are large and extensive, which
is not the case among earthworms, and the former are as a rule more
extensive in the number of segments that they occupy than among the
terrestrial forms. Another difference which they show is that their
cavity is quite simple and not divided up by trabeculae into numerous
intercommunicating chambers as in the earthworms. Finally the eggs of
the aquatic Oligochaeta are large and full of yolk and thus contrast
with the very small ova of the earthworms which are moreover much
more abundant. These features are either of general or universal
occurrence and together form an assemblage of characters which mark
out the aquatic families of Oligochaeta from their, at least mainly,
terrestrial allies.
We may also refer to certain structures which although not universal
among these aquatic families are nevertheless found only in
them--that is, are not found in any family of the terrestrial worms
of this order. The most salient of such characters are the long and
hair-shaped setae tapering to a fine point and often provided with
a series of delicate branches like a feather; such setae are often
of very great length and they occur in their various modifications
among the Aeolosomatidae, Naididae, and Tubificidae. It is clear
that these delicate setae, though they may not be due in any way to
the aquatic life, are rendered possible by it. To drag such tender
processes through stiff clay would surely break and tear them out.
It may be also mentioned that among the aquatic families as a rule
the intersegmental septa do not show that thickening in some of the
anterior segments of the body which is so general a feature of the
land-dwelling species. Finally it is only among the aquatic forms,
and among them only in the families Aeolosomatidae and Naididae, that
asexual reproduction by budding takes place. Indeed so common and
usual is it in the genera of these families that even yet there are
considerable lacunae in our knowledge of the organs of reproduction
in the said families.
Together with these general similarities are many points of
structural difference among the worms inhabiting ponds, lakes, and
rivers, which allow of their being divided into a number of quite
distinct families.
One of the most distinct of these families and lying in any case
quite at the base of the series is the family Aeolosomatidae which
includes a number of distinct species of delicate and transparent
worms, and in whose integument are embedded round cells bearing a
large brightly coloured oil drop; this is reddish or green in colour,
or--and this more rarely--colourless, but still recognisable as an
oil drop. The green sometimes even verges upon blue on the one side
and yellow on the other, while the red may approach brown or purple.
These worms are assigned for the most part to the genus _Aeolosoma_
which is found in all of the great continents and of which seven
or eight species are known. To a more doubtful genus _Pleurophleps_
are assigned a few small worms which have the general appearance of
_Aeolosoma_, but are without the coloured or colourless oil drops
in the skin. These worms have a very large prostomium which is
ciliated on the lower surface, and the body is not markedly segmented
externally by constrictions or internally by septa. The bristles are
slender and hair-like, but among them are in some species the shorter
stouter bristles bifid at the free tip, which are so general among
the aquatic families of the Oligochaeta. These worms are not uncommon
objects in pools containing weed; and they are to be found usually
crawling among the weed. They consist as a rule of but few segments
to most of which a pair of nephridia belong. The ovaries and the
spermaries are only known in a few forms and appear to be unpaired
and lie respectively in the fifth and sixth segments. There are 1-3
pairs of spermathecae, and the sperm ducts if distinct from, are at
least very like, nephridia. The ova appear to make their way to the
exterior by a large aperture in the ventral middle line of a middle
segment of the body. The vascular system contains uncoloured blood
and is greatly simplified.
The next family to the Aeolosomatidae in zoological position is
clearly the Naididae. These are also small worms, but show in some
respects a higher grade of organisation than their allies. While
asexual generation is general, the reproductive organs are more
commonly found than in _Aeolosoma_, though there are still many
hiatus in our knowledge of the same in certain genera. Where they are
known it has been found that the spermaries and ovaries are very far
forward in the body, in the fifth and sixth segments respectively.
The spermathecae are in segment six and the male ducts open into a
terminal chamber, called 'atrium,' which is on the whole not unlike
that of the Tubificidae. The blood in these worms is red as in the
higher types, and thus differs from that of the genus _Aeolosoma_.
The setae are rather varied, being in some cases long and slender,
sometimes greatly exaggerated in length as in _Ripistes_; other setae
are forked at the free end, and in _Paranais_ this is the only kind
of setae met with. A marked feature of this family is that the dorsal
bundles of setae do not always begin like the ventral setae upon the
second segment of the body. Indeed in _Schmardaella_ there are no
bundles of dorsal setae at all. The Indian genus _Branchiodrilus_
is remarkable for the fact that it has paired processes of the body
which may be termed gills and which in some segments involve the
dorsal setae. Another kind of gill is found in the genus _Dero_
(which has many species) and in the allied _Aulophorus_. These
are placed round the vent, and are ciliated. Other genera are
_Nais_, _Chaetogaster_, _Vejdovskyella_, _Amphichaeta_, _Stylaria_,
_Macrochaetina_, _Pristina_, _Naidium_.
Several genera, _Pristina_, _Nais_, _Dero_, are found in many parts
of the world; but it is not possible at present to consider very
seriously the facts of their geographical distribution.
Next to the Naids a group of aquatic worms present themselves for
consideration which are usually placed in three distinct families,
which families are however rather hard to define. These three
families are the _Tubificidae_, _Phreodrilidae_, and _Lumbriculidae_.
The Phreodrilidae were at one time placed with the Tubificidae by
Michaelsen. It is now perhaps the general opinion that they form a
family of their own, at any rate since the discovery of two other
genera _Phreodriloides_ and _Astacopsidrilus_, besides the original
genus founded by myself, and named _Phreodrilus_ from the fact that
the species was found in a deep well in New Zealand.
The distribution of this family especially of the genus _Phreodrilus_
is extremely interesting. The genus _Phreodrilus_ is the only one
genus of the aquatic Oligochaeta which has, like _Notiodrilus_, a
circumpolar range, the pole being the south pole. It occurs in New
Zealand, in Kerguelen, and, if we are to accept the opinions of Drs
Michaelsen and Benham that my genus _Hesperodrilus_ is to be merged
in _Phreodrilus_, in Patagonia also.
In this genus the male pores are upon the XIIth segment while the
spermaries lie in segment XI. Moreover the spermathecae lie behind
the male pores. In the Tubificidae on the other hand it is at least
the rule for the spermaries and male pores to be pushed a segment
further forwards. And the spermathecae are before the male pores.
_Phreodriloides_ is like _Phreodrilus_ but has no spermathecae.
It is also New Zealand in range. _Astacopsidrilus_ is Australian
and is semi-parasitic upon the Crayfish _Astacopsis_, whence its
generic name. _Phreodrilus branchiatus_ is one of the few forms of
Oligochaeta that possesses gills. Of these there are a series of
pairs on about the last eleven segments of the body. They are lateral
in position, and thus contrast with the also gilled _Branchiura
sowerbii_, where the gills, also on the posterior segments of the
body, are more numerous and lie dorsally and ventrally, a pair to
each segment.
The Tubificidae differ from the Phreodrilidae mainly in the points
already noted. There are a considerable number of genera of which
the following are the best known, viz., _Tubifex_, _Limnodrilus_,
_Limnodriloides_, _Branchiura_, _Lophochaeta_, _Ilyodrilus_,
_Psammoryctes_, _Clitellio_, _Telmatodrilus_, _Bothrioneuron_,
_Lycodrilus_.
The Tubificidae are mainly northern temperate forms, and a few of
them such as _Clitellio arenarius_ and '_Peloryctes inquilina_[1]'
are found on the sea coast. There are also a few of this family in
the southern hemisphere. These forms include _Clitellio abjornseni_
from Australia, and a few species of _Branchiura_ from New Zealand
and the islands of the Antarctic ocean. There is also to be mentioned
_Rhizodrilus_ (or _Vermiculus_) _aucklandicus_ from the island of
that name in the New Zealand area. The only tropical species appears
to be the Indian and Malayan _Bothrioneuron iris_, though this genus
also occurs in Europe and in southern South America. It is quite
likely however that _Branchiura sowerbii_, a species known at present
from tanks in hot houses, may be a tropical American species.
[1] With many synonyms, including _Tubifex ater_ (see p. 53).
The family Lumbriculidae is yet more restricted in its range. It has
not yet been met with away from the temperate northern hemisphere,
and the great variety of species recently described from Lake Baikal
by Dr Michaelsen is a very remarkable fact. The Lumbriculidae are
entirely fresh water in habit and not even partially terrestrial.
The following are the principal known genera: _Lumbriculus_,
_Trichodrilus_, _Rhynchelmis_, _Phreatothrix_, _Claparedilla_,
_Stylodrilus_, _Mesoporodrilus_, _Sutroa_, _Eclipidrilus_,
_Aurantina_, _Athecospermia_, _Lamprodrilus_, _Teleutoscolex_.
In the worms of this family the male pores are usually upon the tenth
segment but sometimes upon the eighth or eleventh. And in addition to
a pair of funnels in the antecedent segment there is also commonly a
second pair in the same segment as that which contains the external
pore. The two sperm ducts however open into the same distended atrial
cavity before opening on to the exterior. In _Lamprodrilus_ however
each sperm duct opens by its own separate atrium on to the exterior
and in two consecutive segments. In _Teleutoscolex_ there is but one
pair of funnels opening into the same segment with the atrial pore.
Near perhaps to the Lumbriculidae comes a very small family indeed,
that of the Alluroididae. So small is it that it consists of but
a single genus _Alluroides_ of which there are two species _A.
pordagei_ and _A. tanganyikae_. Both of these species were described
by myself, one of them from but a single specimen, the other from
only two. Both species--and the name of one denotes the region--are
from eastern tropical Africa. The Tanganyika worm is purely aquatic;
the other species was found in a swamp of the mainland opposite to
Mombasa. This remarkable genus has a pair of spermaries in segment
X; but the ovaries are as in earthworms in the XIIIth segment. The
male pores are upon that segment also, _i.e._ further back than
in the aquatic worms. The spermathecae open close to the median
dorsal line of the body in one species; in _A. tanganyikae_ there is
but one spermatheca which opens actually in the dorsal median line
between segments VIII and IX. This family seems to come nearest to
the Lumbriculidae but has also obvious points of likeness to the
terrestrial Moniligastridae. It fully deserves a separate family,
which was founded for it by Dr Michaelsen.
Not obviously related to any of the other families of Oligochaeta is
the family Enchytraeidae. This consists of a very large number of
species, which are placed in about a dozen genera, and whose habitat
is nearly confined to the cold and temperate regions of the world. A
large number of species for example have been described by Dr Eisen
from Alaska, while others inhabit the verge of Patagonia. It is only
a few which are found in warmer countries. There is for instance a
solitary Enchytraeid in India and the neighbouring parts of Asia
described by myself as _Henlea lefroyi_ but doubtfully of that genus
according to Dr Michaelsen. I have also myself obtained another
Enchytraeid from British Guiana. Apart from such rare exceptions the
family is arctic or temperate in its range and is even met with upon
the ice of glaciers. These little worms--they are very rarely of more
than minute size--are both aquatic and terrestrial and inhabitants of
the sea shore. They have four bundles of short often straight and
somewhat stumpy setae; _Anachaeta_ is entirely without setae. That
they bear some relation to the lowest group of Oligochaeta, that of
the Aeolosomas and Naids, appears to be shown by the very anterior
position of the spermathecae, _i.e._ in the fourth or fifth segment.
The spermaries and ovaries on the other hand are in segments XI and
XII. They are peculiar among the aquatic families in having complex
glands appended to the oesophagus which recall the calciferous glands
of the earthworms. The funnel of the sperm duct is extraordinarily
deep and lined with glandular cells except in an apparently primitive
genus from Lake Baikal.
The remaining family of the Limicolae is that of the _Haplotaxidae_
which contains two genera, viz., Haplotaxis, better known as
_Phreoryctes_, and _Pelodrilus_. These two genera overlap somewhat
in their characters and it is in the present state of our knowledge
rather difficult, if indeed possible, to differentiate them
thoroughly. They are slender worms which frequent either damp
earth or water and are thus somewhat intermediate in habit between
the Limicolae and the Terricolae. The chief peculiarity of their
structure lies in the fact that the sperm ducts are unprovided with
any kind of terminal apparatus whatever, but open directly upon
the exterior. Moreover there are generally two pairs of testes in
segments X and XI, and in some species two pairs of ovaries in the
two following segments. The small family is very widely distributed
in more temperate regions, principally of the antarctic hemisphere.
It occurs for instance in New Zealand, South Australia, the Cape, and
in the northern hemisphere in Europe, Western Asia, and North America.